Theme selection to the project description markdown preview

[YoavTC]

Please confirm the following.

• [x] I checked the existing issues for duplicate feature requests
• [x] I have checked that this feature request is not on our roadmap

What parts of Modrinth is your feature request related too?

Modrinth.com website

Is your suggested feature related to a problem? Please describe.

No response

Describe the solution you'd like

A theme selection dropdown menu next to the preview toggle which displays the preview how a person with selected theme would see it.

Here's a quick mock-up:

Describe alternatives you've considered

No response

Additional context

No response